iMac 21,5 Late 2013 A1418 (EMC 2638) Display not working no 4th Diagnostic LED

Hello,
I'm trying to repair an iMac A1418 (EMC 2638) which have a display fault.
Seems on cold boot display works fine but after a while begins to flickering to a black screen; the time span is from few minutes to many hours before the display fails.
I tried an external monitor and it works, correctly.
The board is a 820-3588-A .

When the display is active and alive, I think all values are ok:
Diagnostic LED: #1 ON #2 ON #3 ON #4 ON

J8100
LED_RETURN_1 4,5V (sometimes 11,5V)
LED_RETURN_2 4,5V (sometimes 11,5V)
LED_RETURN_3 4,5V (sometimes 11,5V)
LED_RETURN_4 4,5V (sometimes 11,5V)
LED_RETURN_5 4,5V (sometimes 11,5V)
LED_RETURN_6 4,5V (sometimes 11,5V)
BKLT_BOOST_1 61,3V
BKLT_BOOST_2 61,4V

L440
PP12V_LCD_F 12,1V
PP12V_LCD 12,1V

When suddenly the display goes black, I have:
Diagnostic LED: #1 ON #2 ON #3 ON #4 OFF

J8100
All 0V

L440
PP12V_LCD_F 12,1V
PP12V_LCD 12,1V

VSYNC Cable with no sign of shorts.

Any hint?
Thanks

nobody can help me?

Remove D4450 and replace C4451 with a jumper (temporarily). If screen stopped missing picture, the LCD panel needs to be replaced.

Thank you!
Following your instructions, the panel working the same: fine at cold boot, after few minutes starts to faint to black again.

I changed the panel with the a new one, all good now.
Thanks!

But, it is possible to repair the LCD board?